Influence of Temperature and Environment on the Fatigue Mechanisms of Single-Crystal and Polycrystal 316L

Single-crystal and polycrystal 316L specimens were fatigued at constant plastic strain amplitudes in the range 10−4 to 2 × 10−2 in air and in vacuum (about 3 × 10−4 to 10−3 Pa) at 20, 300, and 600°C. The temperature and strain dependencies of the single-crystal cyclic stress-strain response are qualitatively similar to those of the polycrystalline material.

In air, the fatigue lives of both polycrystals and single crystals decrease by about an order of magnitude at 600°C compared with the results at 20 and 300°C. The fatigue lives of polycrystal specimens were prolonged in vacuum (compared with air) by factors of ∼2 or 3 at room temperature, to ∼20 at elevated temperatures and low plastic strains. The crack initiation mechanism of polycrystals tends to become partially intergranular at 600°C, but this change is not thought to significantly affect fatigue life. The reduction in fatigue resistance in air of both single and polycrystals at 600°C is attributed to oxidation—enhanced Stage I crack initiation.